Joining of an uncured secondary material to an existing cured composite typically requires large assemblies and relatively thick and/or thin individual composite parts, which may then be joined together. This joining method may provide a weaker, secondary joint, which may be difficult to validate for strength and durability using standard detection methods. A reoccurring problem in such secondary joining is that the secondary bond may not be completely laminated or may be a weak bond and the compounded structure may tend to fail or degrade due to mechanical fatigue, impact, or aging. This problem is of great concern due to the widespread use of composite materials used in the aerospace industry. Existing solutions include hand sanding to form an abraded surface, using a dry peel ply, as well as multi-stage curing processes or secondarily bonded assemblies, which may require specific time controls and additional adhesives to secure the bond.
Accordingly, those skilled in the art continue with research and development efforts in the field of composite joining to improve joining of uncured materials to cured composite structures.